The present invention relates generally to an anti-skid brake control system for an automotive vehicle, which includes a pressure reduction power servo system for reducing braking pressure during anti-skid brake control operation. More particularly, the invention relates to the control of a fluid pump as the power servo relative to a wheel speed, especially in a low wheel speed range.
Published Japanese Pat. No. 49-32494, published on Aug. 30, 1974 discloses an anti-lock brake control system having a power servo system which recirculates pressurized fluid in a wheel cylinder when released. The power servo system comprises an electric motor and a fluid pump driven by the electric motor. In such anti-skid control systems, it is considered effective to drive the fluid pump while the anti-lock or anti-skid control system is active.
In general, the anti-lock systems become active when wheel deceleration exceeds a set threshold. Assuming the electric motor with the fluid pump starts operating whenever the wheel deceleration exceeds the set threshold, a problem will arise at relatively low vehicle speeds, e.g. lower than 10 km/h. In such cases, the accuracy of the wheel speed sensor is lowered, increasing systematic error in the detected wheel speed. Since the wheel deceleration is derived from the wheel speed sensor signal value, the wheel deceleration derived from the sensor signal containing such errors will naturally be inaccurate. Thus the wheel deceleration will occasionally be perceived to exceed the set threshold erroneously, unnecessarily activating the anti-lock system and thus the electric motor.
Such spurious operation of the anti-lock system could dangerously extend the braking distance of the vehicle. In addition, the vehicle battery is loaded wastefully by the electric motor driving the fluid pump. Thus, repeated mis-activation of the anti-lock system could prove to be a safety problem and/or a serious drain on the battery.